Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Blown spark plug

Hey guys,
My son has a 03 mustang GT. He called and said it was making a loud clicking sound. I investigated and it appeared to have a exhaust leak, but I couldn’t feel where it was coming from. I started tearing down until I got to the plugs. One of them blew out if the head. I was shocked. Have you heard of this in a car? I’d heard the Ford trucks had problems with the plugs blowing out. Its fixed and running like a champ now.

I’ve seen spark plugs blow out of all kinds of engines.

If a spark plug lossens up, and the combustion pressure keeps pounding on it. Until the plug gets loose to a point where the combustion pressure rips the thread out of the head.


Or someone in the past used the ham fist method of installing new plugs.

@ok4450‌ I actually changed the plugs myself several years ago. Anti-seize and torqued. I hope it wasn’t something I did. :confused:

This is EXTREMELY common on Ford’s modular V8 and V10 engines

There are MANY aftermarket kits out there to install plug repair kits

I’ve even done a few at work

It has little to do with ham fisted mechanics, and more to do with Ford’s design

I am stating my opinions, based on my own experience, and the fact that I’ve seen this phenomenon first hand

For what it’s worth anyway, I’ve been driving 4.6 modulars for about 15 years with never a problem through a godawful lot of miles; even with plugs coming out about every 30k miles on average.
This Mustang should use the same old garden variety plug (Autolite 104) that my Lincoln uses and which has been around forever.

My book shows a torque spec of 7-15 Ft. Lbs and to me that’s ridiculous both as to the spread and the upper amount. Seven Ft. Lbs should be fine in an aluminum head; 15 may well rip the threads out of a tapered seat aluminum head. Who knows; maybe the factory wrenched them down a bit much when they built the car back then. At some point weakened threads can let go for whatever reason.


“plugs coming out about every 30K . . .”

That might be part of the reason why they haven’t blown out on your personal engines

My camry has an all aluminum engine, and Toyota calls for 16 ft-lbs. Should I question that?

That wasn’t a serious question, by the way

In fact, I recently changed the plugs on my engine. I torqued them all to 16 ft-lbs, as per Toyota

For me, the ham fisted approach would be using a 1/2" drive long ratchet or breaker bar, or perhaps a 1/2" drive air impact wrench

Some of you will be pleased to know that I used the plugs mentioned in the owner’s manual. And I checked the gap on all of them. They were all perfect. No gapping needed.

@ok4450‌ 7-15 ft-pd is a wide range. I’ll check my manual tomorrow and see if its advising those #'s. Memory has me thinking it was hard number, but I can’t remember what that number was.

Glad you got it fixed, did you have to re-tap the head? Install a thread insert?

You may get differing opinions here, but my advice would be to never use anti-seize on spark plugs. It’s just not needed, and causes more problems than it supposedly solves. How strongly do I feel about that? A few years ago I had a guy working here that insisted on putting anti-seize on spark plug threads, despite repeated requests from me to never do so. He continued the practice and I finally threw out the bottle.

@asemaster‌ I had it towed to a local shop. They fixed it for slightly more than what I would have had in the kit to re- thread and install the insert.
Why is anti-seize a no no? I changed plugs in my compact tractor and I’m sure they didn’t use it when installing the plugs. I was sweating bullets getting one of the stuck plugs out. Do you think the anti-seize contributed to the plug blowing out? The plug still had the film on it. I’m sure the plugs have been in for at least two years.

I’ve heard that anti-seize acts as a lubricant and throws off the reading if you’re using a torque wrench. You end up putting the plug in tighter than it should be.

@jdmere is correct about anti-seize

If you put a lot of antiseize on a lug bolt and attempt to torque it, you will bottleneck the threads, because you are actually overtorquing it

I know this firsthand

So putting too much antiseize on a spark plug would also cause problems

Anti-seize can throw off the tightening spec when installing spark plugs, whether using a torque wrench or the experienced “Gudenteit” method. You can over tighten or under tighten, the anti-seize can interfere with the heat dispersion from the spark plug body to the cylinder head. If there were any reason to use anti-seize on spark plugs they would come that way from the factory (with a very few exceptions). Use the correct spark plugs installed properly and there is no need for anti-seize. Spark plugs seizing in a head is simply not a problem.

@asemaster‌ I’ll take that into account on my next plug change. Hopefully I’ll be able to do it. :slight_smile:

You should check the plug gap before installation. The spec on my 4.6s has been .054 but many come out of the box at .045-.046 and that’s a bit much to let slide.

Regarding my prior comment about snugging them up being a better option than a torque wrench figure which has a wide spread, if you look on the package the plugs come in (say Autolite) you will not find a torque figure on there.
What you will find is (paraphrased) “Run them down finger tight. Finish tightening with an additional 1/4 to 1/2 turn.”.